Refresh customization docs

[brkalow]

🔎 Previews:

What does this solve?

• Clarifies the different layers of Clerk customization so readers can choose the right approach instead of hunting through scattered content
• Moves the Account Portal guide out of the customization section

What changed?

• Added a Customizing Clerk overview plus a dedicated Bring Your Own CSS guide, which moves existing content from the appearance prop overview page to a more use case focused guide
• Reworked the appearance prop reference to point toward the new guides and trim redundant explanations
• Relocated account-portal/overview to its own guide folder and updated the docs manifest and redirects to match the new structure

Checklist

• I have clicked on "Files changed" and performed a thorough self-review
• All existing checks pass

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Project	Deployment	Preview	Updated (UTC)
clerk-docs	Ready	Preview	Nov 22, 2025 3:33am

[github-actions]

⚠️ TypeDoc files detected in this PR

This PR modifies files in the 'clerk-typedoc/' folder. These files are auto-generated from the clerk/javascript repository and should not be edited directly.

To make changes to TypeDoc documentation:

1. 🔄 Make your changes in the appropriate files in the clerk/javascript repository.
2. 🚀 The TypeDoc documentation will be pulled through to this repository via a CI action.

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